1. Field of the Invention
The present invention relates to an image forming apparatus for a printer or copier which forms a color image through an electrophotographic process and a method thereof, and more particularly to an image forming apparatus involving an intermediate transfer process in which toner images of different colors formed on plural photosensitive drums are transferred to an intermediate transfer belt in a way for transferred images to overlap each other and the resulting image is finally transferred onto paper, and a method thereof.
2. Description of the Related Art
Conventionally image forming apparatuses such as printers which use an electrophotographic process to form color images are roughly classified into two types: the four-pass type and the single-pass (tandem) type.
FIG. 1 shows the process of a conventional four-pass system. The four-pass system has a single photosensitive drum 100 and a developing unit 106 for forming four color images: yellow (Y), magenta (M), cyan (C) and black (K) images. The surface of the photosensitive drum 100 is evenly charged by a charger 102 located after a cleaning blade 101 and an electrostatic latent image is formed through laser scanning by an exposure unit 104. Then, yellow toner in the developing unit 106 develops the photosensitive material on the drum to make the latent image appear and the yellow toner image is transferred to an intermediate transfer belt 108 which is in contact with the photosensitive drum 100. This toner transfer is electrostatically made by a transfer roller 110 which applies transfer voltage. After this, the same procedure is repeated for magenta, cyan and black toners in the order of mention so that the four color toner images are laid one upon another on the transfer belt 108. Finally a transfer roller 111 transfers the four color developers (toners) at a time onto paper and the resulting image is fixed by a fixing device 112.
Therefore, the four-pass system just requires one set of the following components for the intermediate transfer process: the photosensitive drum 100, cleaning blade 101, charger 102, exposure unit 104 and transfer roller 110. In this sense, the system is advantageous in terms of cost. However, the intermediate transfer belt 108 must be rotated four turns to make a single color image, which means that the speed of color printing is one fourth the speed of monochrome printing.
FIG. 2 shows the process of a single-pass type (tandem type) system (Japanese Published Unexamined Patent Application No. Hei 11-249452, etc). In the single-pass type system, image forming units 112-1 through 112-4 for yellow (Y), magenta (M), cyan (C) and black (K) are aligned in a row. The image forming units 112-1 through 112-4 respectively have photosensitive drums 114-1 through 114-4 around each of which a cleaning blade, a charger, an LED exposure unit and a developing device are located, and the image forming units 112-1 through 112-4 respectively form images of different colors. The images of different colors formed on the photosensitive drums 114-1 through 114-4 are electrostatically transferred in sequence to an intermediate transfer belt 116 moving in contact with the photosensitive drums 114-1 to 114-4, in a way to overlap each other as transfer voltage from transfer rollers 118-1 to 118-4 is applied to the belt; and finally the finished image is fixed on paper by a fixing device 122.
When a transfer belt is used as an intermediate transfer means as in this case, generally the process of transferring (and overlapping) images from the photosensitive drums to the intermediate transfer belt is referred to as primary transfer while the process of transferring four color images at a time from the intermediate transfer belt to paper is referred to as secondary transfer. Generally speaking, the transfer rollers 118, which are used for primary transfer, and a paper transfer roller 120 which is used for secondary transfer are both conductive sponge rollers. The primary transfer rollers and the secondary transfer rollers are respectively located opposite to the photosensitive drums and to a backup roller, with the intermediate transfer belt between them.
In this single-pass type system, a color image is obtained through a single-pass, so the printing speed is faster than in the four-pass type system. However since the single-pass type system requires an image forming unit and a transfer roller for each color, it is more costly.
In addition, the intermediate transfer rollers must have prescribed electric resistance, sponge hardness and sponge surface precision. Further, the intermediate transfer components are not treated as consumable like image forming units and their replacement period is relatively long, which means they must be electrically and mechanically durable enough. One approach to reducing cost and enhancing reliability may be to use metal intermediate transfer rollers. However, if metal rollers should be in pressure contact with the photosensitive drums through the intermediate transfer belt, the transfer nip as the point of contact between the photosensitive drum and the transfer belt would become unstable, resulting in local transfer failures. For this reason, it has been almost impossible to use metal rollers.
Furthermore, in a system which uses an intermediate transfer belt and a paper conveyer belt, sponge leavings from sponge transfer rollers may adhere to the rear face of the belt or the belt drive roller surface and thus cause slippage between the belt and the drive roller, resulting in serious image defects such as color alignment errors and jitter.
Another problem in the intermediate transfer process of the single-pass type system is that the time of primary transfer voltage application may coincide with the time of secondary transfer voltage application and the power supply to apply secondary transfer voltage may be turned on during primary transfer. In some such cases, the secondary transfer voltage (current) interfered with the primary transfer process through the intermediate transfer belt as a resistor, leading to an image defect such as streaks.
In the single-pass type system, as illustrated in FIG. 2, the intermediate transfer rollers 118-1 to 118-4 and the photosensitive drums 114-1 to 114-4 constitute a primary transfer section while the paper transfer roller 120 and the backup roller, which face each other with the intermediate belt transfer 116 between them, constitute a secondary transfer section; and as illustrated in FIG. 3, the volume resistance in the direction of the thickness of the intermediate transfer belt 116 is used for transfer. However, the volume resistance of the intermediate transfer belt 116 and the transfer voltage largely depend on each other as indicated in a result of measurement in FIG. 4 so transfer is apt to be unstable. Especially, when the transfer belt has considerably deteriorated over time, transfer image blurring often occurs. As the transfer voltage to be applied to the transfer rollers increases, the resistance of the transfer belt decreases and so there occurs much current leakage from the belt area other than its transfer area corresponding to the paper width, causing a problem such as loss of current or a failure to transfer an image onto paper with a small width.